To capture an open Gym for your team:
- Walk up to a gray-colored Gym.
- Touch the Gym to enter it.
- At the bottom of the screen, touch Add Pokémon
- Select a Pokémon to add to the Gym.
You can only add one Pokémon per Gym. Your Pokémon won't be returned to your collection until it's knocked off the Gym, so choose carefully!